Biocathodes for the reduction of the highly toxic hexavalent chromium (Cr(VI)) were investigated using Shewanella oneidensis MR-1 (MR-1) as a bio-catalyst and performance was assessed in terms of current production and Cr(VI) reduction. Potentiostatically controlled experiments (-500 mV vs. Ag/AgCl) showed that a mediatorless MR-1 biocathode started up 2 under aerated conditions in the presence of lactate, received 5.5 and 1.7 times more electrons for Cr(VI) reduction over a 4-hour operating period than controls without lactate and with lactate but without MR-1, respectively. Cr(VI) reduction was also enhanced, with a decrease in concentration over the 4-h operating period of 9 mg/L Cr(VI), compared to only 1 and 3 mg/L respectively in the controls. Riboflavin, an electron shuttle mediator naturally produced by MR-1, was also found to have a positive impact in potentiostatically controlled cathodes. Additionally, a microbial fuel cell (MFC) with MR-1 and lactate present in both anode and cathode produced a maximum current density of 32.5 mA/m 2 (1,000 Ω external load) after receiving a 10 mg/L Cr(VI) addition in the cathode, and cathodic efficiency increased steadily over an 8-day operation period with successive Cr(VI) additions. In conclusion, effective and continuous Cr(VI) reduction with associated current production were achieved when MR-1 and lactate were both present in the biocathodes. INTRODUCTIONHexavalent chromium (Cr(VI)) is a highly toxic, mutagenic and carcinogenic substance that is present in the effluent streams of a wide range of industrial processes, including electroplating, leather tanning and wood preserving 1 . It is highly soluble and, because of its long history of use and often its disposal after inappropriate or no treatment, it has become one of the most abundant inorganic contaminants in groundwater. Ideally Cr(VI) should be removed from groundwater in the natural environment, and because of the inherent dangers to health the stringent guideline limit of 50 μg/L has been issued for total chromium concentration in drinking water 1,2 . Removal options include ion-exchange, adsorption and electrodialysis 3 ; however in many of these applications chromium keeps its toxic hexavalent state. Reduction of Cr(VI) to the considerably 3 less toxic trivalent form Cr(III) and its subsequent precipitation at neutral pH could be considered a more effective remediation strategy 4 .A possible method for Cr(VI) reduction using a microbial fuel cell (MFC) has recently been proposed, where Cr(VI) was used as an oxidant in the cathode to generate an electrical energy output 5,6 . At low pH values where H + is abundant, cathodic Cr(VI) reduction has been demonstrated at relatively fast rates and without the use of a catalyst 5,6 . In the neutral pH region, however, the reaction kinetics are slower due to low H + availability:CrO 4 2-+ 8H + + 3e -= Cr 3+ + 4H 2 O (1) At neutral pH, Cr(VI) reduction takes place under lower cathode potentials and is also severely inhibited by Cr(III) oxyhydroxide monolayers w...

Please cite this article in press as: N. Xafenias, et al., Electrochemical startup increases 1,3-propanediol titers in mixed-culture glycerol fermentations, Process Biochem (2015), http://dx. a b s t r a c tIn this study we investigated the use of electric potential to bioelectrochemically ferment glycerol, a cheap by-product of biodiesel production, into valuable 1,3-propanediol (1,3-PDO). The 1,3-PDO production rates were increased up to 6 times in electrofermentations, compared to non-electrochemical fermentations, and high concentrations up to 42 g 1,3-PDO/l were achieved in fed-batch mode. Extensive growth of the well-known 1,3-PDO producers Clostridiaceae (55-57%) was observed when an appropriate potential (−1.1 V vs. SHE) was constantly applied since the start. Potential propionate producers (Veillonellaceae) were also among the dominant families (20-21%); however, surprisingly enough, propionate production was not observed. On the contrary, Clostridiaceae were absent, Veillonellaceae dominated (56-72%), and propionate was produced when electric potential was not sufficient for current production since the beginning. In all cases, glycerol consumption ceased and electrocatalytic activity was lost when we replaced the biofilm electrodes with electrodes lacking a biofilm, clearly demonstrating that glycerol electrofermentation was mostly supported by the bacteria located in the biofilm. In the nonelectrochemical systems the performance and the titers achieved were poor; only 18 g 1,3-PDO/l was achieved in more than twice the time, and lactate producing Lactobacillaceae became dominant.

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The work investigated the efficiency of microbial fuel cells (MFCs) for the treatment of alkaline hexavalent chromium containing wastewater. When lactate was used as the metal chelator in alkaline (pH 8) abiotic cathodes, hexavalent chromium concentration dropped from 10 mg l -1 to undetectable levels within the first 45 h of operation. Power density produced in the pH 8 abiotic cathodes was up to 21.4 mW m -2 , and in the pH 9 cathodes up to 2.4 mW m -2 ; these values were well comparable with other values found in the literature for biologically catalysed cathodes, even at lower pH values. When Shewanella oneidensis MR-1 was present in a hexavalent chromium reducing cathode at pH 8, current production contributed by 26 % to the total hexavalent chromium reduced during the 36 days of operation. On the other hand, when hexavalent chromium (10 mg l -1 ) was controllably added in the anode where S. oneidensis MR-1 was present, up to 73 % of current decreased immediately after every hexavalent chromium addition; this toxic effect remained even after hexavalent chromium was depleted in the anode and strongly indicates that the presence of hexavalent chromium in the anodes of MFCs must be avoided. Overall, our results indicate that alkaline hexavalent chromium wastewater can be effectively remediated in the cathodes of MFCs, provided that a metal chelator is present in the cathodes and that hexavalent chromium is not present in the anodes.

It has been suggested that application of electric potential can affect lysine producing fermentations, although experimental evidence is lacking. To study this hypothesis we used the lysine producer Corynebacterium glutamicum ZW04, and we exposed it to 12 different conditions regarding anaerobic gas environment, applied electrode potential (cathodic, open circuit, anodic), redox mediator and nitrate presence. The gas environment was found to play a major role, with CO leading to double the lysine concentrations and yields when compared to N. Electrode potentials also played a major role, with reductive conditions doubling the titers and increasing the yields of lysine up to 4 times. Addition of the redox mediator anthraquinone-2-sulfonate (AQ2S) under the presence of CO and reductive conditions led to additional doubling of the titers, although the yields were not altered considerably. This study demonstrates for the first time that cathodic electrode conditions combined with CO and AQ2S as a redox mediator can significantly improve both the yields and the titers of lysine production of a C. glutamicum lysine producing strain, reaching levels that have only been achieved under aerobic conditions.